Die mounting substrate and method of fabricating the same

ABSTRACT

In accordance with various embodiments, there is provided a method of fabricating a die mounting substrate, including the steps of preparing a mounting substrate including a pad and a die including a terminal, and printing a conductive paste bump on one of the pad or the terminal. The method further includes the step of connecting the pad and the terminal to each other using the conductive paste bump, thereby surface-mounting the die on the mounting substrate.

RELATED APPLICATION

This application is a divisional application of U.S. patent applicationSer. No. 12/714,077, filed on Feb. 26, 2010, and claims the benefit ofand priority to Korean Patent Application No. KR 10-2009-0114719, filedon Nov. 25, 2009, all of which are incorporated herein by reference intheir entirety into this application.

BACKGROUND

1. Field of the Invention

The present invention relates to die mounting substrate and a method offabricating the same.

2. Description of the Related Art

With the recent reduction in size and fabrication cost of electronicapparatuses, electronic components such as semiconductor chips orpackages for molding the semiconductor chips, including BGA, CSP, WLPand so on, are surface-mounted on a mounting substrate by a flip chipbonding process using a solder base material (solder ball).

However, the flip chip bonding process using solder balls is difficultto apply when a mounting substrate has the same size as that of anelectronic component. In particular, in the case where soldering isperformed using a typical solder paste having an average particle sizeof 5˜20 μm, solder balls have a diameter of 105˜120 μm, undesirablymaking it difficult to cope with fineness of a circuit pattern. Also,because solder balls subjected to reflow have a spherical shape, theadhesion area of the solder balls to the circuit pattern is reduced,undesirably weakening an interfacial adhesion force and bringingadjacent solder balls into contact with each other. As well, in order tosolve problems with reliability resulting from different coefficients ofthermal expansion between the mounting substrate and the electroniccomponent, shear stress should be relieved at the connection portions.In the case where the height of solder balls is increased, shear stressat the connection portions may be relieved. However, limitations areimposed on increasing the height of solder balls which use a solder basematerial.

With the goal of solving the above problems, there have been proposedmethods of forming copper pillars on the mounting substrate orelectronic component through plating and photolithography and thenprinting solder balls thereon, but the plating and photolithographyincrease the process costs and are complicated, undesirably lowering theprocess yield.

In addition, although attempts to use a double solder ball structure arebeing made, problems of interfacial adhesion force of the solder ballsto the circuit pattern being reduced due to fineness of the circuitpattern and of adjacent solder balls coming into contact with each otherstill occur.

SUMMARY

Accordingly, the present invention has been made keeping in mind theproblems encountered in the related art and the present invention isintended to provide a die mounting substrate in which shear stress isrelieved thus preventing reliability from decreasing due to a differencein the coefficient of thermal expansion between a die and a mountingsubstrate and also preventing a force of adhesion of bumps fromdecreasing due to the fineness of a circuit pattern, and a method offabricating the same.

In accordance with at least one embodiment, there is provided a diemounting substrate, including a mounting substrate having a pad, a diehaving a terminal and surface-mounted on the mounting substrate, and aconductive paste bump formed on the pad or the terminal so as to connectthe pad and the terminal to each other.

In accordance with an embodiment, the conductive paste bump has the samesize as the pad or the terminal on which the conductive paste bump isformed.

In accordance with another embodiment, one side of the conductive pastebump has the same size as the pad, and the other side of the conductivebump has the same size as the terminal.

In accordance with another embodiment, the conductive paste bump has thecross-sectional shape of a trapezoid or an inverted trapezoid.

In accordance with another embodiment, the conductive paste bumpincludes copper, silver, or carbon nanotubes.

In accordance with another embodiment, an underfill resin is appliedbetween the mounting substrate and the die.

In accordance with at least one embodiment, there is provided a methodof fabricating the die mounting substrate, including (A) preparing amounting substrate having a pad and a die having a terminal, (B)printing a conductive paste bump on the pad or the terminal, and (C)connecting the pad and the terminal to each other using the conductivepaste bump, thus surface-mounting the die on the mounting substrate.

In accordance with an embodiment, the conductive paste bump is printedby means of a screen printer using a conductive paste to have apredetermined height through continuous printing.

In accordance with another embodiment, one side of the conductive pastebump has the same size as the pad, and the other side of the conductivebump has the same size as the terminal.

In accordance with another embodiment, the conductive paste bump has thecross-sectional shape of a trapezoid or an inverted trapezoid.

In accordance with another embodiment, the conductive paste bumpincludes copper, silver, or carbon nanotubes.

In accordance with another embodiment, the method further includes (D)applying an underfill resin between the mounting substrate and the die,after (C).

Various objects, advantages and features of the invention will becomeapparent from the following description of embodiments with reference tothe accompanying drawings.

BRIEF DESCRIPTION OF DRAWINGS

These and other features, aspects, and advantages of the invention arebetter understood with regard to the following Detailed Description,appended Claims, and accompanying Figures. It is to be noted, however,that the Figures illustrate only various embodiments of the inventionand are therefore not to be considered limiting of the invention's scopeas it may include other effective embodiments as well.

FIGS. 1A and 1B are cross-sectional views showing die mountingsubstrates, in accordance with embodiments of the invention.

FIGS. 2 to 5 are cross-sectional views sequentially showing a process offabricating a die mounting substrate, in accordance with embodiments ofthe invention.

FIG. 6 is a schematic top plan view showing a screen printer used forbump printing, in accordance with an embodiment of the invention.

FIG. 7 is a schematic cross-sectional view showing a printing portion ofthe screen printer of FIG. 6, in accordance with embodiments of theinvention.

DETAILED DESCRIPTION

The present invention will now be described more fully hereinafter withreference to the accompanying drawings, which illustrate embodiments ofthe invention. This invention may, however, be embodied in manydifferent forms and should not be construed as limited to theillustrated embodiments set forth herein. Rather, these embodiments areprovided so that this disclosure will be thorough and complete, and willfully convey the scope of the invention to those skilled in the art.Like numbers refer to like elements throughout. Prime notation, if used,indicates similar elements in alternative embodiments.

Die Mounting Substrate

FIGS. 1A and 1B are cross-sectional views showing die mountingsubstrates, in accordance with embodiments of the invention. Withreference to these drawings, the die mounting substrates 100 a, 100 baccording to the embodiments of the present invention are describedbelow.

As shown in FIGS. 1A and 1B, the die mounting substrate 100 a, 100 baccording to the present embodiment is configured such that pads 132 ofa mounting substrate 130 and terminals 112 of a die 110 are connected toeach other using conductive paste bumps 120 and the die 110 issurface-mounted on the mounting substrate 130. Specifically, in thepresent embodiment, the die 110 is connected and mounted on the mountingsubstrate 130 not using solder balls but using conductive paste bumps120.

The die 110 is configured such that the terminals 112 are formed on theupper surface of a wafer chip body made of silicon and including anintegrated circuit (not shown) therein so as to form an electricalconnection to the integrated circuit, and a passivation layer is formedon the upper surface of the wafer chip body to expose the terminals 112.

The conductive paste bumps 120 are used to connect the terminals 112 ofthe die 110 and the pads 132 of the mounting substrate 130 to eachother, and are provided between the terminals 112 of the die 110 and thepads 132 of the mounting substrate 130.

As such, the conductive paste bumps 120 are formed to have the same sizeas the terminals 112 or the pads 132 on which they are formed, thusenlarging the adhesion area to thereby enhance the force of adhesion.This is possible because the conductive paste bumps 120 are formed byprinting a conductive paste using a screen printing process, without anadditional reflow process. The formation method thereof is describedbelow.

For example, as shown in FIG. 1A, in the case where the conductive pastebumps 120 are formed on the terminals 112 of the die 110 and are mountedin inverted form on the mounting substrate, the conductive paste bumps120 have the cross-sectional shape of an inverted trapezoid, and oneside of the conductive paste bumps 120 is formed to have the same sizeas do the terminals 112. As shown in the cross-sectional view, thediameter D1 of one side of the conductive paste bumps 120 formed on theterminals 112 is formed to be the same as the width W1 of the terminals112. Typically, the terminals 112 of a die 110 are smaller than the pads132 of a mounting substrate. Thus, in the case where the conductivepaste bumps 120 are formed to have the cross-sectional shape of aninverted trapezoid on the terminals 112, the diameter D2 of the otherside of the conductive paste bumps 120 is smaller than the width W2 ofthe pads 132.

As shown in FIG. 1B, in the case where the conductive paste bumps 120are formed on the pads 132 of the mounting substrate 130 and are mountedon the mounting substrate 130, the conductive paste bumps 120 have thecross-sectional shape of a trapezoid, and one side of the conductivepaste bumps 120 in contact with the pads 132 is formed to have the samesize as do the pads 132. As shown in the cross-sectional view, thediameter D1 of one side of the conductive paste bumps 120 formed on thepads 132 is formed to be the same as the width W2 of the pads 132.Because the diameter D2 of the other side of the conductive paste bumps120 having the trapezoidal shape is formed to be smaller than thediameter D1 of one side thereof, the diameter D2 of the other side ofthe conductive paste bumps 120 in contact with the terminals 112 is madeto be the same as the width W1 of the terminals 112, thus maximizing thearea of contact between the terminals 112 and the conductive paste bumps120.

In accordance with at least one embodiment, the conductive paste bumps120 is made of any conductive material, for example, copper (Cu) orsilver (Ag) paste. In the case of the Ag paste, because the Ag particleshave an average diameter of about 1˜3 μm, it is possible to print theconductive paste bumps 120 having a diameter of about 80 μm, thus copingwith fineness of the terminals 112 or the pads 132.

In addition, as shown in Table 1 below, the conductive paste bumps 120,according to an embodiment of the invention, includes carbon nanotubeshaving much higher electrical properties compared to a metal materialsuch as aluminum or copper which is superior in terms of electricalconductivity and specific resistance, thus improving interlayerconnection reliability and increasing heat dissipating efficiency.

TABLE 1 Physical property Carbon Nanotubes Comparative Materials Density1.33~1.40 g/cm³ 2.7 g/cm³ (Al) Current density 1 × 10⁹ A/cm² 1 × 10⁶A/cm² (copper cable) Thermal 6000 W/mK 400 W/mk (copper) conductivitySpecific 1 × 10⁻¹⁰ Ωcm 1 × 10⁻¹⁰ Ω 

 (copper) resistance

The mounting substrate 130 indicates a printed circuit board on whichthe die 110 is mounted. The configuration thereof is not particularlylimited, and any configuration may be used so long as pads 132 areformed on an insulating layer.

In order to improve reliability of the connection of the conductivepaste bumps 120 for connecting the die 110 and the mounting substrate130 and to protect the substrate from the external environment, anunderfill resin 140 is applied between the die 110 and the mountingsubstrate 130.

Fabrication of Die Mounting Substrate

FIGS. 2 to 5 are cross-sectional views sequentially showing the processof fabricating the die mounting substrate, in accordance withembodiments of the invention. With reference to these drawings, themethod of fabricating the die mounting substrate according to thepresent embodiment is described below.

As shown in FIG. 2, conductive paste bumps 120 are printed on theterminals 112 of a wafer chip 110 a.

As such, the conductive paste bumps 120, in accordance with at least oneembodiment, is printed on the terminals of the wafer chip 110 a using ascreen printing process for printing bumps in which conductive paste istransferred via a mask having openings. Specifically, the screenprinting process is performed by applying the conductive paste on theupper surface of the mask and then pressing the conductive paste using asqueegee so that the conductive paste is transferred onto the terminalsvia the openings of the mask, and enables the formation of bumps havingthe desired height and shape by controlling the number of continuouscycles of a screen printer, the printing speed of a screen printer, theviscosity and thixotropy of a conductive paste, and the aspect ratio ofa mask.

The conductive paste bumps 120 are formed such that the diameter D1 ofone side of the conductive paste bumps 120 connected to the terminals112 is greater than the diameter D2 of the other side thereof, so thatthe whole assumes, for example, the cross-sectional shape of atrapezoid. Because the conductive paste bumps 120 are formed such thatthe one side of the conductive paste bumps 120 connected to theterminals 112 has the same shape as the terminals 112, that is, D1=W1,the area of contact is enlarged thus enhancing the force of adhesion ofthe conductive paste bumps 120 to the terminals 112.

In accordance with at least one embodiment, the height of the conductivepaste bumps 120 is increased by controlling the number of continuouscycles of a screen printer, and the viscosity and thixotropy of theconductive paste. For example, in the case where the conductive pastehas a viscosity and thixotropy adapted for high printability, the numberof continuous cycles of the screen printer is increased, thus increasingthe height of the conductive paste bumps 120. In addition, the shape ofthe conductive paste bumps 120 is varied by controlling the printingspeed of the screen printer, the viscosity and thixotropy of theconductive paste, and the aspect ratio of the mask. As such, as theshape of the conductive paste bumps 120 is controlled, the area ofcontact with the terminals is increased, thus improving reliability ofadhesion.

Next, as shown in FIG. 3, the wafer chip 110 a is cut along the dicingline DL using a dicing device, thus preparing the die 110 on which theconductive paste bumps 120 are printed on the terminals 112.

Next, as shown in FIG. 4, the conductive paste bumps 120 are connectedto the pads 132 of the mounting substrate 130, thereby surface-mountingthe die 110 on the mounting substrate 130.

Finally, as shown in FIG. 5, an underfill resin 140 is applied betweenthe die 110 and the mounting substrate 130 using a dispensing processand is then cured, thus completing the die mounting substrate 100 a.

The method of fabricating the die mounting substrate 100 a of FIG. 1A isdescribed as above, and may be applied to the fabrication of the diemounting substrate 100 b of FIG. 1B, as is apparent to those skilled inthe art. Specifically, the conductive paste bumps 120 are formed on thepads 132 of the mounting substrate 130, the die 110 is mounted thereon,and the underfill resin 140 is applied, thus fabricating the diemounting substrate 100 b as shown in FIG. 1B.

Screen Printer

FIG. 6 is a top plan view schematically showing a screen printer usedfor printing the bumps, in accordance with an embodiment of theinvention. FIG. 7 is a cross-sectional view schematically showing theprinting portion of the screen printer of FIG. 6, in accordance withembodiments of the invention. The screen printer 200 is described belowwith reference to these drawings.

As shown in FIGS. 6 and 7, a screen printer 200 is moved in thedirections of X, Y, Z and θ, and includes a printing table 210 on whicha printing member 220 is mounted through vacuum adsorption, a mask 230which is installed in a non-contact state on the printing table 210 andhas a plurality of openings 232 for printing the bumps, and a squeegee240.

The mask 230 includes plate lifters 234 a, 234 b for lifting the sideportions of the mask 230 upward so that the spread of bumps is minimizedthus maintaining the shape of bumps and also so that the printed bumpsare not brought into contact with the mask thus improving the quality ofthe printing, depending on the direction of movement of the squeegee240. As such, the plate lifters 234 a, 234 b include a first platelifter 234 a and a second plate lifter 234 b provided at both sides ofthe mask 230 in order to enable the separation of the plate along thedirection of movement of the squeegee 240. For example, in the casewhere the squeegee is moved in a first printing direction (to the rightin the drawing), the first plate lifter 234 a lifts one side of the mask230 upward so that the printed bumps are not brought into contact withthe mask 230. In contrast, in the case where the squeegee is moved in asecond printing direction (to the left in the drawing), the second platelifter 234 b lifts the other side of the mask 230 upward so that theprinted bumps are not brought into contact with the mask 230.

The squeegee 240 functions to press the paste into the openings of themask so that the bumps are printed on the printing member 220, andincludes a squeegee holder 242 and a squeegee rubber 244 removablymounted to the squeegee holder 242. The squeegee 240 is movedupward/downward and rightward/leftward by a squeegee driving portion(not shown).

As described hereinbefore, the present invention provides a die mountingsubstrate and a method of fabricating the same. According to the presentinvention, a die is connected and mounted on a mounting substrate usingconductive paste bumps, thus relieving shear stress, and therebypreventing the reliability from decreasing due to a difference in thecoefficient of thermal expansion between the die and the mountingsubstrate and also preventing the force of adhesion of bumps fromdecreasing due to the reduction in size of the pads formed on themounting substrate.

Also, according to the present invention, it is easy to control theheight and shape of the conductive paste bumps for connecting the dieand the mounting substrate, thus coping with the fineness of a circuitpattern and increasing the height of bumps, thereby relieving shearstress at the connected portions.

Embodiments of the present invention may suitably comprise, consist orconsist essentially of the elements disclosed and may be practiced inthe absence of an element not disclosed. For example, it can berecognized by those skilled in the art that certain steps can becombined into a single step.

The terms and words used in the present specification and claims shouldnot be interpreted as being limited to typical meanings or dictionarydefinitions, but should be interpreted as having meanings and conceptsrelevant to the technical scope of the present invention based on therule according to which an inventor can appropriately define the conceptof the term to describe the best method he or she knows for carrying outthe invention.

The singular forms “a,” “an,” and “the” include plural referents, unlessthe context clearly dictates otherwise.

As used herein and in the appended claims, the words “comprise,” “has,”and “include” and all grammatical variations thereof are each intendedto have an open, non-limiting meaning that does not exclude additionalelements or steps.

Ranges may be expressed herein as from about one particular value,and/or to about another particular value. When such a range isexpressed, it is to be understood that another embodiment is from theone particular value and/or to the other particular value, along withall combinations within said range.

Although the present invention has been described in detail, it shouldbe understood that various changes, substitutions, and alterations canbe made hereupon without departing from the principle and scope of theinvention. Accordingly, the scope of the present invention should bedetermined by the following claims and their appropriate legalequivalents.

What is claimed is:
 1. A method of fabricating a die mounting substrate,comprising: (A) preparing a mounting substrate comprising a pad and adie including a terminal; (B) printing a conductive paste bump on one ofthe pad or the terminal; and (C) connecting the pad and the terminal toeach other using the conductive paste bump, thereby surface-mounting thedie on the mounting substrate.
 2. The method as set forth in claim 1,wherein the conductive paste bump is printed by means of a screenprinter using a conductive paste to have a predetermined height throughcontinuous printing.
 3. The method as set forth in claim 1, wherein oneside of the conductive paste bump comprises a size same as the pad, andthe other side of the conductive bump comprises a size same as theterminal.
 4. The method as set forth in claim 1, wherein the conductivepaste bump comprises a cross-sectional shape of a trapezoid or aninverted trapezoid.
 5. The method as set forth in claim 1, wherein theconductive paste bump comprises copper, silver, or carbon nanotubes. 6.The method as set forth in claim 1, further comprising: (D) applying anunderfill resin between the mounting substrate and the die, after (C).